XI – How to Find the Oxidation Number of Central Atom – Step-by-Step Guide for Class XI Chemistry

Learn how to find the oxidation number of the central atom in any compound or ion with simple rules, formulas, and solved examples. Perfect for Class XI Chemistry students.

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In this post, we’ll learn how to find the oxidation number of the central atom in a given species. Understanding oxidation numbers is an important concept in Redox Reactions and Chemical Bonding. You’ll also see step-by-step examples to make it easy for you to calculate oxidation numbers quickly and correctly. 

🧪 Steps to Find Oxidation Number

1. Write the formula of the compound or ion.

2. Assign oxidation numbers to known atoms using standard rules.

3. Let the oxidation number of the central atom be x.

4. Form an equation where the sum of oxidation numbers = total charge.

5. Solve for x to find the oxidation number of the central atom.

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⚖️ Important Rules

Atom/Group	Oxidation Number
Hydrogen (H)	+1 (–1 in metal hydrides)
Oxygen (O)	–2 (–1 in peroxides, +2 in OF₂)
Alkali metals (Na, K, etc.)	+1
Alkaline earth metals (Ca, Mg, etc.)	+2
Halogens (Cl, Br, I)	–1 (except when bonded to O or F)
Sum of oxidation numbers	0 (for neutral) / equal to ion charge

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🔹 Examples

Example 1:H2SO4

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Let oxidation number of S = x

→ 

$2(+1) + x + 4(-2) = 0$

→ x = +6

Example 2:

$MnO_4^-$

Let oxidation number of Mn = x

→ 

$x + 4(-2) = -1$

→ x = +7

Example 3:

$Cr_2O_7^{2-}$

Let oxidation number of Cr = x

→ 

$2x + 7(-2) = -2$

→ x = +6

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🧠 Quick Tip:

Always remember:

The total oxidation number of all atoms = charge on the molecule or ion.

XI Finding out Oxidation Number of Central atom in a given specie

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